The present invention relates to method and apparatus for measurement of fluid thermal conductivity utilizing a steady state hot-wire method, wherein thermal conductivity of fluids being tested are obtained on the basis of a differential temperature between an electrically heating sensor arranged in thermal contact with the fluids to be measured and the fluids during heat generation from the heating sensor.
In general, the thermal conductivity of potential process fluids such as liquid mixture of liquid and solids or gas is one of the most important factors to be properly monitored and/or controlled throughout the production facilities in various industries since the thermal conductivity of a fluid largely depends on temperature, composition, mixing condition of ingredients and other factors. Furthermore it is essentially to measure the thermal conductivity directly on the production facilities in view of the fact that the thermal conductivity is one of the physical properties that is unpredictable from known additive properties. Particularly in a process involving an operation of heating and/or cooling, the optimal control conditions for such heating and/or cooling will vary and, in consequence, significantly affect the product quality as a change occurs in the thermal conductivity. Accordingly, it is essential for the process control to measure the thermal conductivity and thereby to correspondingly change the condition of control.
Additionally, if the concentration of a composition of substances such as emulsion can be determined on the basis of a change in the thermal conductivity, the in-line control of said concentration will be drastically facilitated and therefore the thermal conductivity will be one of the most important factors to be measured in such case.
Practice in the measurement of the fluid thermal conductivity by the unsteady state hot-wire method is well known, for example, from the following literature:
1. U. Nagasaka and A. Nagashima: "Study on Measurement of Fluid Thermal Conductivity with High Precision," Transactions of the Japan Society of Mechanical Engineers, Vol. 47, 417 (May, 1981), pp. 821-829;
2. U. Nagasaka and A. Nagashima: "Study on Measurement of Fluid Thermal Conductivity with High precision", Transactions of the Japan Society of Mechanical Engineers, Vol. 47, 419 (July, 1981), pp. 1323-1331; and
3. "Handbook of Thermophysical Properties", edited by Japanese Society of Thermophysical Properties, May 30, 1990, published from YOKENDO, pp. 568-573
The method for measurement of fluid thermal conductivity is generally classified into the unsteady state hot-wire method and the steady stale hot-wire method.
The unsteady state hot-wire method utilizes a time-depending change appearing in a temperature of the heating element in unsteady state in which the temperature rises as time passes, starting immediately from the initiation of heat generation to determine a thermal conductivity of the fluid to be measured. The steady state hot-wire method, on the other hand, utilizes a steady state temperature field, i.e. , performs the measurement at a time-independent constant temperature reached after said unsteady state has been stabilized.
In general, the steady state hot-wire method is apt to be affected by a convective heat transfer due to convection caused by a temperature rise of the fluid to be measured and it will be impossible to determine the thermal conductivity with high precision unless the affect of the convection is eliminated. In contrast with the steady state hot-wire method, the unsteady state hot-wire method is advantageous in that the adverse effect of the convection can be reliably eliminated by detecting the onset of the convection and utilizing the value obtained prior to said generation of the convection.
This is why the unsteady state hot-wire method has been often employed in practice to measure a thermal conductivity of fluid.
Both above-identified literature references 1 and 2 report typical embodiments of the unsteady state hot-wire method, for example, a metallic thin wire placed in sample fluid in a vertical orientation is energized and a thermal conductivity is calculated based on the heating value and the temperature of the filament thus energized. Literature reference 3 describes details of both the steady state hot-wire method and the unsteady state hot-wire method.
The present invention is relevant particularly to a so-called concentric cylinder method as an embodiment of the steady state hot-wire method as described in the literature reference 3. With this concentric cylinder method, fluid to be measured is introduced into the clearance defined between an outer and an inner cylinders and the temperature of the fluid in the clearance is measured by a plurality of thermocouples while a heating element contained within the inner cylinder along a central axis thereof is energized for heat generation.
The unsteady state hot-wire method for measurement of fluid thermal conductivity as described in the above-identified literature uses platinum thin wire whose diameter is less than 50 microns in order to improve the precision of the measurement. Accordingly, actual operation of the measurement is conducted on separately provided samples of fluid and cannot be used for in-line measurement within a plant. This is true also with respect to the steady state hot-wire method described in literature reference 3.
In other words, no attempt has been made to realize an in-line measurement of the fluid thermal conductivity on actual site of production.
For measurement of the thermal conductivity using the steady state hot-wire method, a convection heat transfer possibly occurring in the fluid around the heating element must be reliably avoided. Otherwise, undesirable heat movement would be caused by a convective heat transfer, resulting in a value of the measured thermal conductivity substantially higher than an apparent effective thermal conductivity calculated on the basis of a heat movement caused only by conductive heat transfer in a static state of the fluid.
The above-identified literature references exemplarily report the method of measurement in which the fluid to be measured is confined in a clearance defined between the heating element and a spacer means. However, such method neither assumes a convection of the fluid possibly occurring in the clearance nor considers an influence of such convection on the measurement. Further, such an apparatus used to perform such method is too complicated to be incorporated in a production line, and even if such incorporation is possible, would inevitably encounter a problem such as washability.
The typical apparatus with tile concentric cylinder method that is conventionally known has been disadvantageously complicated and expensive since the apparatus comprises a plurality of thermometers contained within a cell particularly made of silver to achieve a uniform temperature distribution of the sampled fluid.
Japanese patent application Disclosure Gazettes Nos. 1989-180444 and 1991-17542 describe high precision measuring methods for thermal conductivity of fluid utilizing the unsteady state hot-wire method. The method utilizing the unsteady state hot-wire method as disclosed in the former Disclosure Gazette No. 1989-180444 takes account of a measurement error due to, an electric resistance appearing in a bridge used to read-out a signal output from the sensor.
The method described in the latter Disclosure Gazette No. 1991-17542 determines the thermal conductivity based on a linear relationship established between a temperature rise and a period of energization in order to eliminate an influence of convective heat transfer occurring in the fluid on the measurement utilizing the unsteady state hot-wire method.
These two prior techniques are basically different from the steady slate hot-wire method according to the present invention in that they adopt the unsteady state filament heating method. The unsteady state hot-wire method had an intrinsic drawback such that a value obtained from direct measurement must be processed to determine a thermal conductivity. Particularly, the hot-wire method described in the Disclosure Gazette No. 1989-180444 involves various factors that must be taken into account for determination of the thermal conductivity such as changes in resistance as well as in temperature and a range of temperature.
It is also difficult for these two prior techniques to be effectively incorporated in production lines because of their intrinsic drawbacks such that the measurement is batch-based, the metallic thin wire employed has a poor resistance against vibration and the apparatus itself is readily affected by a change in the environmental temperature.